It is possible to add a kinetic mixing term as the \(g_{VB}\)  term in Eq. () to the flipped model I that was considered in Sec. . This was done in Ref. [1]}, where the coupling constant \(g_{VB}\)  was denoted by \(\delta \) . In principle, such models have two parameters in the dark vector sector: a kinetic mixing, as in the secluded model in Sec. , and a bare Stueckelberg mass \(M_B\)  as in the flipped I model in Sec. .
